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>> Monday, August 23, 2010

Relax Max continued: If I may have two questions, the second is about terminal velocity. If a large object (a passenger airliner, say) reaches terminal velocity before impact, do smaller (or more aerodynamic) objects whose terminal velocity are higher (I assume) "pass" the airliner on the way down? Or does everything hit the ground at the same time?

Good question(s). As you asked, items don't have the same terminal velocity. The maximum speed an item attains in air depends on many different factors. Surface area, fluid factors, and mass are key factors, but other factors include roughness, shape, initial speed, etc. A man falling, for instance, can change his terminal velocity drastically between falling spread eagle and pulling his limbs in and falling headfirst (as skydivers do to move up and down relative to each other). A man can't change it enough not to be going too fast for landing without a parachute, but that's a different post.

Terminal velocity is effectively the speed an object obtains when the force of gravity is canceled by the opposite drag on an object so that it stops accelerating. If gravity were not involved, there'd be no terminal velocity because drag would just work to make things go slower with no counterforce. (Newtonian physics I can explain if you'd like). Initial velocity makes a difference because it adds a factor beyond gravity (and velocity has an effect on drag).

A debris field is determined by multiple factors as well: initial speed, what caused the initial breakup, and how much and what kind of debris is generated. A biplane, for instance, that lost it's rudder might have a very limited debris field, where as Columbia's debris field extended over several states. Explosions (whether combustive or pressure built) send debris forward and backward, extending the debris field. Flat low mass debris will fall slower that compact debris.

So, to answer the last, everything doesn't land at the same time - except in a vacuum as they demonstrated on Apollo 15.

5
comments:

I recall learning about terminal velocity as a part of parachute training.Our trainer was very persuasive, he explained that, without a parachute, a human body in free fall will not normally exceed about 125mph.If you roll into a ball, you might reach about 200mph. People who try very hard to fall quickly, (yes, there are some out there) by falling head or feet first, in carefully streamlined posture, can get the speed up to about 320 mph.Joseph Kittinger, who you have mentioned before, was believed to have hit a maximum speed of slightly over 600mph. But that was up in the stratosphere, where humans rarely venture, unless they're on the way into or out of orbit, and virtually never outside of a space capsule of some sort.Rob, our ex-military crazy-man trainer.... would then push us out of the back of a moving truck, to practise our landings. The point was not lost on us. 15 mph hurts quite enough if you get the landing wrong.At freefall speeds, all our efforts to slow down, if we forget to properly deploy the canopy, will only serve to vary the depth of the crater.

Yes, one doesn't need a parachute to jump out of an airplane, one only needs one to survive that jump. Paraphrasing one of your quotes of sometime back.

Anyway, thank you for answering my questions. What you said about decompression makes sense, and I even suffered through the part about the poor diver exploding then having his parts sucked through a small opening. I do agree (how could I not?) that decompression of an airliner alone does not have to be catastrophic or even deadly. In my case, neither airliner was flyable due to the damage done by the collision, not by the resulting decompression.

Joseph Kittinger fascinates me. In many ways I'm more overawed by what he did than I am of the early capsule astronauts. A few years ago I read a book by a man who had decided to break Kittinger's record.He was Charles 'Nish' Bruce, writing under the pseudonym 'Tom Read'. The book follows the writer's career as a soldier, SAS special forces high altitude parachutist, first special forces soldier parachuted into the Falklands, during the war with Argentina, to his fascination with the ultimate high altitude jump, russia spacesuits etc.But he further writes of what was to stop his attempt at the record, mental illness.His fellow soldiers rallied around to try help him, but his constant fear was that he was too dangerous to be around those he loved. Ultimately, he took his own life. Falling out of the sky.http://www.guardian.co.uk/g2/story/0,3604,630819,00.htmlThe book shows the price our military too often pay for the things that are done at the whim of governments in our name.http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=freefall+tom+read&x=0&y=0&ih=2_2_1_1_2_0_1_0_0_1.16_139&fsc=-1